The present invention relates to a process for the preparation of terephthalic acid by oxidizing p-xylene in the liquid phase.
The oxidation of p-xylene into terephthalic acid in the liquid phase by molecular oxygen and in the presence of a heavy metal catalyst is a matter of considerable industrial importance and tremendous amount of work has been devoted to this problem over the past three decades. The major difficulty which is encountered when carrying out this operation results from the fact that, although p-xylene is easily transformed into p-toluic acid, the further oxidation of the latter is much more difficult and only negligible yields of terephthalic acid are obtained when ordinary catalytic processes are used. Numerous methods have been devised to overcome this difficulty, most of which consist in adding some activator or promoter. One of these methods comprises the use of a bromine-containing compound as an activator and a lower fatty acid, e.g., acetic acid, as a solvent. Although this method has reached full commercial fruition, it suffers serious drawbacks since the use of bromine at high temperatures raises severe corrosion problems which can only be solved by using expensive, highly corrosion-resistant equipment such as Hastelloy C or titanium. Moreover, under the strong oxidizing conditions applied in this process, the acetic acid solvent is significantly consumed which results in additional costs.
To avoid those corrosion problems, other methods have been proposed which consist in using as an activator, instead of bromine, an aldehydic and/or a ketonic compound, e.g., acetaldehyde and/or methylethylketone. These methods require less severe conditions and, although acetic acid is still used as a solvent, conventional stainless steel equipment can be employed. However, the activator is consumed in the reaction, mainly by oxidation into acetic acid which is therefore a co-product of the reaction and must be separated, purified and sold for the process to be economically attractive. Still other methods have been proposed to avoid the drawbacks arising from the use of an extraneous compound as an activator. For instance, it has been shown possible to oxidize in good yield p-xylene into terephthalic acid in an acetic acid medium containing only cobalt as a catalyst but in exceedingly large amounts. Nevertheless, in this case also, a significant consumption of acetic acid takes place.
More recently, different methods have been claimed whereby p-xylene is oxidized into terephthalic acid in the absence of any solvent and activator. Temperatures higher or close to the melting point of p-toluic acid, i.e., 179.degree. C., are used in order to achieve liquid-phase conditions. Although these methods appear as being remarkably simple in their principle, they are difficult to apply in practice: without an activator the intermediate p-toluic acid is rather refractory to oxidation and without solvent, technical problems arise which are associated with the handling of solids and the removal of the reaction heat. For instance, the separation of terephthalic acid from the other components of the reaction mixture is a difficult task which is generally achieved by heating and washing treatments at elevated temperatures, e.g., at 230.degree.-270.degree. C. as is disclosed in U.S. Pat. No. 3,883,584 or even at 290.degree.-350.degree. C. as in U.S. Pat. No. 3,711,539. Obviously, such treatments require the use of expensive pressure vessels in corrosion-resistant materials and should cause increased decomposition and coloration of the reaction products.